1. Field of the Invention
The invention relates generally to electronic devices and more particularly to protection circuits for protecting loads from voltage sources.
2. Background
Many electronic devices have electric circuits, known as sensitive circuits, that require protection from certain problematic electrical conditions, such as voltage and power conditions. Common problematic conditions include high voltages (above some threshold), negative voltages and fast changing, negatively moving voltages, also referred to as fast negative transients.
Examples of devices that commonly have circuits that require protection include portable electronic equipment such as, for example, personal digital assistants, laptop computers, and wireless communication devices, including at least cell phones and pagers. Commonly, the sensitive circuits need protection from other circuits to which the sensitive circuits may be connected. For example, AC and DC power sources may present problematic electric conditions to the sensitive circuits. The power source may also be a car power adapter. Another source of problematic electrical conditions is failed AC to DC adapters. Some failed AC to DC adapters may fail such that they present an AC output at the DC side of the adapter.
The numbers and types of electronic devices and their adapters and connections to other electronic devices are growing constantly. It is sometimes troublesome for consumers to be certain that they are connecting the correct connector (e.g., power supply) into their electronic devices. For example, a consumer may have more than one cell phone and more than one charger for each cell phone. The consumer does not want the cell phone to break if the wrong charger is mistakenly plugged into one of the cell phones. Alternately, cell phone manufacturers do not want their cell phones to break if a consumer mistakenly connects the cell phone to the wrong charger. Several companies make cell phone chargers, known as after-market chargers, for other companies' cell phones. The cell phone manufacturers would like to be as certain as possible that the after-market chargers do not break their cell phones, even considering that the after-market chargers may not contain adequate protection circuits.
Some known solutions include connecting the source to the load through a diode, as shown in FIG. 12. The diode 200 is connected to a voltage source 210 and a load 220. The FIG. 12 circuit typically results in poor energy transfer (e.g., charging) efficiency. Diodes have a non-zero forward voltage VF. They therefore consume energy, lower the voltage available to the load, and produce heat.
Another known solution is shown in FIG. 11. The FIG. 11 circuit also uses a diode 230, but in this case, the diode is connected between ground and the input 240 of the load 220. A fuse 250 is connected between a voltage source 210 and the load 220. A problem with the FIG. 11 circuit is that when an excess current condition occurs, the fuse becomes a permanent open circuit and the source 210 is no longer connected to the load.
Some solutions do not provide for protection from fast negative shoot-through. That is, problems occur if an input voltage drops quickly. Fast negative transients can cause negative shoot through. Negative shoot through happens when a fast negative transient results in a negative voltage appearing on the load side of a device. For example, a protection circuit may be designed to prevent negative voltages from passing across the protection circuit to a load. A negative shoot through occurs when a transient negative voltage reaches the load due to a fast changing negative input voltage.